1) Technical Field of the Invention
The present invention relates to a complex type of an image forming apparatus performing various functions as a printer, a copier, and a facsimile machine, and in particular, relates to the image forming apparatus capable of switching a color mode for reproducing a color image and a monochrome mode for forming a monochrome image.
2) Description of Related Arts
In general, a color image forming apparatus has the color mode for forming the color image and the monochrome mode for reproducing the monochrome (black and white) image, which can be switched. Also, the color image forming apparatus is well known to have two types of driving engines such as a four-cycle engine and a tandem engine, for reproducing the color image.
The image forming apparatus with the four-cycle engine includes a rotatable developing rack for receiving four developing devices each containing respective one of color toner of yellow, magenta, cyan, and black.
In a color mode operation of the image forming apparatus with the four-cycle engine, by rotation of the developing rack, each of the four developing devices received therein is arranged to oppose to an image carrier (or photoreceptor) in sequence. The image carrier is developed or receives the image thereon with the first toner supplied from the developing device firstly opposing to the image carrier. Then, the toner image on the image carrier is primarily transferred onto an intermediate transfer member (or an intermediate transfer belt). Similarly, three of other color toners, in turn, are primarily transferred onto the intermediate transfer member so that four color toner images are superimposed thereon. Then, the superimposed color images are secondarily transferred from the intermediate transfer member onto a paper.
In a monochrome mode operation of the image forming apparatus with the four-cycle engine, the developing rack rotates to arrange solely the developing device containing the black toner to oppose the image carrier, and then only the black toner image is primarily transferred thereon, which in turn is secondarily transferred onto the paper.
In the meanwhile, the image forming apparatus with the tandem engine includes four imaging units, each of which are arranged in straight along the intermediate transfer member, including the image carrier and the developing device.
In the color mode operation of the image forming apparatus with the tandem engine, similar to the four-cycle engine, the imaging units develop the toner images of yellow, magenta, cyan, and black, respectively, which are primarily transferred onto the intermediate transfer member in sequence for overlapping those toner images, and then secondarily transferred therefrom onto the paper at a time.
In the monochrome mode operation of the image forming apparatus with the tandem engine, only the black toner image is developed, primarily transferred, and secondarily transferred as described above.
The conventional color image forming apparatus has a couple of drawbacks, for example, each color image is not precisely registered to the other color images (which is referred to as “out-of-color-registration”). Also, a cleaning blade of the image carrier, which removes the toner remained thereon even after primarily transferring the toner image, may have a short lifetime by readily wearing away during limited operation. Therefore, a color image forming apparatus has been demanded, which avoids the out-of-color-registration and causes the cleaning blade to be less worn (abraded) away.
With regard to the former problem, if the image carrier has a circumferential speed (tangential speed of an outer surface thereof) set to be the same as that of the intermediate transfer member, the actual circumferential speed of the image carrier is slightly deviated from one of the intermediate transfer member so that each of the toner images is misaligned onto the intermediate transfer member, thereby causing the out-of-color-registration.
As suggested in a Japanese patent application, e.g., JP 2002-365874, A, which is referred to as Reference 1, in order to prevent the out-of-color-registration, the circumferential speed of the image carrier is usually set to be slightly offset from that of the intermediate transfer member. This effectively works to eliminate the out-of-color-registration. In the image forming apparatus of Reference 1, a difference of the circumferential speeds between the image carrier and the intermediate transfer member is provided in the color mode, for addressing the out-of-color-registration. Meanwhile, substantially no difference of the circumferential speeds between the image carrier and the intermediate transfer member is set in the monochrome mode.
As to the later drawback, i.e., the abrasion of the cleaning blade for the image carrier (image-carrier cleaning blade), a cohesive mass (agglomeration) is often built up on the intermediate transfer member and shifted therefrom onto the image carrier. The cohesive mass passing through a gap between the cleaning blade and the intermediate transfer member causes the abrasion of the edge of the cleaning blade, and may chip off a portion of the edge of the cleaning blade. The chipped portion of the edge causes the redundant toner on the image carrier that should have been removed off by the cleaning blade, to remain on the image carrier so that the toner image has an undesired line, which requires the cleaning blade to be replaced with a new one.
The aforementioned cohesive mass is believed to be generated by the toner or an external additive of the toner passing through a gap between the intermediate transfer member and the cleaning blade, and/or frictioning with a secondary transfer roller. Also, in the image forming apparatus with the four-cycle engine, since the cleaning blade for the intermediate transfer member (intermediate-transfer cleaning blade) contacts on and off the intermediate transfer member, the cohesive mass may be generated due to such on-and-off operation of the intermediate-transfer cleaning blade. More particularly, the toner accumulated adjacent the edge of the intermediate-transfer cleaning blade may remain on the intermediate transfer member after the cleaning blade is lifted off the intermediate transfer member, and/or the toner mass on the edge of the cleaning blade may drop onto the intermediate transfer member by the shaking (vibration) of the cleaning blade when being lifted off the intermediate transfer member, thereby generating the cohesive mass.
In the meanwhile, the abrasion of the image-carrier cleaning blade is likely accelerated when the developing device is not operated (i.e., no toner is supplied onto the image carrier). Thus, when the developing device is operated with the toner supplied onto the image carrier, the toner or the external additive of the toner on the image carrier is accumulated adjacent to the cleaning blade edge of the image carrier to define a stable layer extending along the cleaning blade edge, which blocks the cohesive mass from passing through the gap in between. However, when the image carrier continues to rotate without being supplied with toner from the developing device (i.e., no operation of the developing device), the stable layer shrinks and disappears due to no supply of the toner or the external additive of the toner. Then, the cohesive mass shifted from the intermediate transfer member likely passes through the gap between the image carrier and the image-carrier cleaning blade, without being blocked by the stable layer. Therefore, it is desirable to continuously operate the developing device in order to prevent the abrasion of the image-carrier cleaning blade.
On the contrary, the continuous operation of the developing device should be avoided since it may deteriorate the toner contained in the developing device. In general, the toner in the developing device is exposed to mechanical and electrical stress in a supply and/or restriction step and may be deteriorated for a long-term operation of the developing device. The toner deterioration causes various problems appeared on the reproduced image such as a fogged toner, a spilled toner, a reduced density of image, and an uneven density of image. Therefore, in order to avoid those problems, it is preferable that the developing device is operated only when forming the image and suspended during the time of no image being reproduced.
The time of no image being reproduced includes, for example, the times while the intermediate transfer member is being cleaned, while the image carrier is being charged and rotated for stabilizing a charging potential before forming the toner image, while the charge on the image carrier is being removed (neutralized) with charging and exposing light after forming the image, and while the developing rack in the four-cycle engine is being rotated.
As such, appropriate steps have to be taken for effectively preventing the abrasion of the image-carrier cleaning blade, especially in case where the image carrier is rotated but the developing device is not operated. This is important especially in the monochrome mode because in the four-cycle engine and the tandem engine of the image forming apparatus, the image-carrier cleaning blade is more likely abraded away in the monochrome mode rather than the color mode, as will be indicated below.
Thus, when the image forming apparatus with the four-cycle engines is used in the monochrome mode, the rotation of the intermediate transfer member that is required for forming the monochrome image is less than (i.e., one-fourth of) the total rotation thereof in number. In other words, the intermediate transfer member and the image carrier rotate while the developing device is not operated (i.e., while no toner is supplied onto the image carrier) in the monochrome mode. To the contrary, in the color mode, the intermediate transfer member and the image carrier rotate with the toner supplied onto the image carrier. Therefore, the image-carrier cleaning blade in the monochrome mode is more likely worn away than in the color mode.
Similarly, as the image forming apparatus with the tandem engine is used in the monochrome mode, since three of imaging units except black one are not driven when forming the black image, the image-carrier cleaning blade of those imaging units are likely bladed away due to lack of the stable layer on the cleaning blade edge.